Process of coating aluminum with other metals



United States Patent Oifice 3,531,379 Patented Sept. 29, 1970 3,531,379 PROCESS OF COATING ALUMINUM WITH OTHER METALS Arthur Leslie Peach, St. Petersburg, Fla., assignor to Micral Industries, Inc., St. Petersburg, Fla., a corporation of Florida No Drawing. Filed July 28, 1965, Ser. No. 475,594 Int. Cl. C23b 3/02, 5/50, 9/02 The portion of the term of the patent subsequent to Feb. 7, 1984, has been disclaimed U.S. Cl. 20433 5 Claims ABSTRACT OF THE DISCLOSURE A process of forming a tenaciously adherent plating on an aluminum object consisting of This invention relates to a process of applying to an aluminum surface a coating or plate of some other metal which is very adherent, and will not blister or peel when the aluminum is brazed or soldered.

For many purposes such as in the missile and space industries aluminum is preferred primarily because of its low weight. It is also an inexpensive metal and can be fabricated easily and inexpensively. Most of its physical characteristics are good but its electrical conductivity is only fair, especially in the transmission of high-frequency alternating currents which are to a considerable extent a surface function, depending on the frequency. The relatively poor conductivity of high frequency currents by aluminum as compared with some other metals is probably due in part to a surface oxidation, often impalpable, which soon forms on aluminum articles exposed to the atmosphere. Surface conductivity of aluminum can be greatly improved to the desired degree for electrical high-frequency transmission by the application of a plating of some other metal such as silver, gold or nickel. However, if the plated object is to be a part of a missile, for example, the plating must be tenaciously adherent or it will be practically useless since a plating failure in a missile invariably means a failure of instrumentation and therefore a total failure of the missile. The plating must therefore in such cases be so adherent that it will not peel or blister when the part is brazed or soldered.

The importance of applying to aluminum objects a strongly adherent plating has long been recognized as indicated, for example, in the US. patent to Travers, No. 1,871,761, granted Aug. 28, 1934. This patent describes a process of forming an anodic film on the surface of an aluminum object, partially removing the anodic film, then electro-depositing a metal on the treated surface. The present invention is an improvement over the methods described in the Travers patent and relates to a method of treating aluminum objects which results in a nickel plating which is so adherent that it successfully withstands severe tests such as direct contact with an oxy-acetylene flame without blistering. The making of a hole through an aluminum sheet which has been plated Cir in accordance with the present invention, by locally melting the sheet, though accompanied by a rippling of the sheet around the hole, failed to cause the plating to separate from the sheet.

According to the present invention, the improved method comprises treating by a special process the aluminum surface to be plated, then subjecting the treated surface to a special electro-electroless plating process such as is described in my copending application Ser. No. 301,- 495, filed Aug. 12, 1963 pursuant to which Pat No. 3,303,111 was granted Feb. 17, 1967.

The following procedure is prescribed for treating the surface of an aluminum object. Such object must be entirely of aluminum as should also accessory elements such as connecting wires, hooks or racks. Such elements should be rigidly attached to the object by a clamp, a screw, or equivalent means. The first step of treatment is to degrease the object in a spirit solvent, trichlorethylene vapor or the like. The second step is to etch the object in a solution of sodium hydroxide (8 ounces to the gallon) for 10 to 20 seconds at a temperature of 180 to 200 F., followed by a rinse. The third step is ot etch the object in an acid solution nitric acid, 25% hydrofluoric acid) for 5 to 15 seconds at a temperature of 60 to F., followed by a rinse. The fourth step is the immersion of the object in an phosphoric acid solution (354 grams per litre, using deionized water) for 10 to 30 minutes (preferably about 20 minutes) at a temperature of 60 to F. While in this bath, a direct current of electricity is passed through the object, a conductor suitable current-carrying capacity being employed to connect the object to the positive pole of the current supply, the negative pole or cathode preferably being the stainless steel lining of the tank holding the electrolyte. Anysuitable source of electric energy may be employed, such as a direct current generator or a rectifier capable of supplying a pressure of 28 volts. When the object has been placed in the electrolyte bath, a low voltage is applied and is slowly increased to 28 volts in a period of from 30 seconds to 1 /2 minutes preferably 1 minute. This voltage is maintained for the rest of the period of immersion. The amperage of the current passed through the object is incidental and may vary widely according to the kind of object being treated.

After this anodizing operation, the object is thoroughly rinsed and then transferred to a plating bath having a pH of 7.09.0 and consisting of: nickel sulphate (and/or chloride) 4 oz. per gallon of water; sodium citrate, 4 oz. per gallon of water; sodium hypophosphite, 2 oz. per gallon of water. Through this bath a direct current of about 2 volts is passed, this voltage being much below that usually considered to be essential for conventional electroplating. A nickel anode should be used having a surface area not more than one-tenth of the surface area of the cathode so that the relatively high current density at the anode will prevent nickel from 'being deposited thereon from the electrolyte. The temperature of the bath should be F. or more.

After from two to five minutes in this bath, the object should be transferred for the remainder of the 20-minute period to a similar bath having a pH of 3.5 to 5.0. If the object is of simple shape and is not a casting, it can be transferred directly from the phosphoric acid bath to the low pH plating bath.

The current density on the cathode surface should be from 0.7 to 3 amperes per square foot of surface. The applied current may be advantageously intermittent or even alternating. If the latter, both electrodes should be work pieces with about the same surface area.

The combination of the preliminary treatment and the electro-electroless plating method herein'before de- 3 scribed results in a nickel coating on aluminum objects which is highly adherent and has uniformly withstood severe tests.

The nature of the treated surface of the aluminum object is not definitely known except that it is a surface efiect on the object and is an integral part thereof, but it is known to resist corrosion and to promote strong adherence of plating metal to the object.

I claim:

1. In the process of depositing a nickel coating on an aluminum article, wherein said article is made cathode in a primary bath of aqueous electrolyte consisting essentially of nickel sulphate and chloride, sodium citrate and sodium hypophosphite in the approximate proporations of 4 oz. of nickel sulphate and chloride, 4 oz. sodium citrate, and 2 oz. sodium hypophosphite in each gallon of water and wherein a current of about 0.73 amperes per square foot of cathode surface area is used, the improvement wherein said article before coating is immersed as an anode in an aqueous acidic bath comprising about 354 g./l. of phosphoric acid and a current is passed therethrough for a period of about -30 min. at a temperature of about 6090 F. to deposit a coating on said article.

2. The process of claim 1 additionally including the step of pretreating said article which comprises immersing said article as a cathode in a pre-treatrnent bath of an aqueous solution consisting essentially of about 4 oz./ gal. nickel sulphate and chloride, 4 oz./ gal. sodium citrate, and 2 oz. sodium hypophosphite, said solution having a pH of about 3.5-5.0, and passing a current of about 0.7- 3 amperes per square foot of cathode area to said article, and subsequently transferring said article as a cathode to said primary bath.

3. The process of claim 1 additionally including the pre-steps of degreasing the surface of the object, etching 4 the object in an aqueous solution of sodium hydroxide for about 10-20 seconds at a temperature of from about 180-200 F., rinsing in water, etching in an aqueous acid solution consisting of nitric acid and 25% hydrofluoric acid for 5-15 seconds at a temperature of from 6080 F., and rinsing in water.

4. The process of claim 1 additionally including the step of applying a low voltage to the phosphoric acid bath and gradually increasing the voltage to about 28 volts within about seconds and maintaining about 28 volts for the remainder of the period.

5. The process of claim 1 wherein said primary bath includes a nickel anode having a surface area of not more than of the surface of the cathode.

References Cited UNITED STATES PATENTS 2,036,962 4/1936 Fischer 20458 XR 2,095,519 10/1937 Fischer 204-42 3,303,111 2/1967 Peach 204-43 XR OTHER REFERENCES Bengston, Helmer, Methods of Preparation of Aluminum for Electrodeposition, transactions of the electrochemical society, pp. 307-319, vol. 88, 1945.

S. Wernick et al., Finishing of Aluminum, p. 262, (1959).

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner U.S.-Cl. X.R. 

